An official website of the United States government.

We've made some changes to If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

Health and Ecological Effects

On this page:

How are humans exposed to cyanobacteria and cyanotoxins?

The most common exposures to cyanobacteria and their toxins during recreational activities are:
  • Oral from accidental or deliberate ingestion of contaminated water;
  • Dermal by direct contact of exposed parts of the body to water containing cyanobacteria cells. Also, cyanobacteria cells can accumulate in bathing suits, particularly diving suits, breaking and exposing skin to toxins, and;
  • Inhalation through the aspiration of water containing cyanobacteria cells and their toxins.
Photo of lake
Cyanobacteria bloom at Grand Lake St. Marys, Ohio, 2010. Photo by Ohio EPA.

Other major routes of human exposure are through ingestion of cyanotoxin-contaminated drinking water, inhalation while showering, dietary intake via consumption of cyanotoxins in contaminated foods and algal dietary supplements, and exposure from water used in medical treatments (e.g., medical dialysis). Consumption of mussels and clams collected during cyanobacterial blooms or immediately after blooms should be avoided.

Wind-driven currents may cause buoyant cyanobacterial blooms to amass on shorelines. These accumulations of cyanobacteria cells are much larger than blooms in open waters, thus presenting a greater risk to human and animal health.

What health risks do humans face as a result of exposure to cyanotoxins?

Adverse health outcomes from exposure to cyanotoxins may range from a mild skin rash to serious illness or death. Acute illnesses caused by exposure to cyanotoxins have been reported and after short-term exposures, microcystin and cylindrospermopsin could cause liver and kidney damage. The table below summarizes the health effects caused by the most common toxin producing cyanobacteria.

Cyanotoxins Acute Health Effects in Humans Most common cyanobacteria producing toxin
 Microcystin-LR Abdominal pain, Headache, Sore throat, Vomiting and nausea, Dry cough, Diarrhea, Blistering around the mouth, and Pneumonia Microcystis, Anabaena, Nodularia, Planktothrix, Fischerella, Nostoc, Oscillatoria, and Gloeotrichia
 Cylindrospermopsin Fever, Headache, Vomiting, Bloody diarrhea Cylindrospermopsis raciborskii, Aphanizomenon flos-aquae, Aphanizomenon gracile, Aphanizomenon ovalisporum, Umezakia natans, Anabaena bergii, Anabaena lapponica, Anabaena planctonica, Lyngbya wollei, Rhaphidiopsis curvata, and Rhaphidiopsis mediterranea
 Anatoxin-a group Tingling, burning, numbness, drowsiness, incoherent speech, salivation, respiratory paralysis leading to death* Chrysosporum (Aphanizomenon) ovalisporum, Cuspidothrix, Cylindrospermopsis, Cylindrospermum, Dolichospermum, Microcystis, Oscillatoria, Planktothrix, Phormidium, Anabaena flos-aquae, A. lemmermannii Raphidiopsis mediterranea (strain of Cylindrospermopsis raciborskii), Tychonema and Woronichinia

* Symptoms observed in animals.

How do you treat people that have been exposed to cyanotoxins?

In the event that you do come into contact with water that is known to be contaminated with cyanotoxins, the Centers for Disease Control (CDC) recommends that you rinse off with clean, fresh water as soon as possible. Seek medical treatment right away if you think you or someone you know might have been poisoned by cyanobacterial toxins, especially when any of the symptoms mentioned above are recognized.

How do cyanobacteria affect aquatic ecosystems?

High biomass blooms, whether of toxic or nontoxic species, can lead to very low oxygen levels in the water column (hypoxia), resulting in higher mortality rates in local fish, shellfish, invertebrate, and plant populations. The blooms may also affect benthic flora and fauna due to decreased light penetration. Toxic blooms from some cyanobacteria genera may lead to inhibition of other phytoplankton and suppression of zooplankton grazing, leading to reduced growth and reproductive rates and changes in community structure and composition.

What other impacts do cyanobacteria have on their environment?

In addition to the production of toxins, cyanobacteria have often been associated in drinking water with taste and odor problems. Dying and lysing cells release their contents (toxins) into the water and are subject to rapid putrefaction of the material. Blooms produce a variety of odor and taste compounds, such as geosmin and 2–methylisoborneol (MIB), which are not toxic but are a nuisance to the public.

More Information

U.S. EPA Managing Cyanotoxins in Public Drinking Water Systems
US EPA Health Effects Support Document for the Cyanobacterial Toxin Anatoxin-a
US EPA Health Effects Support Document for the Cyanobacterial Microcystins Toxins
US EPA Health Effects Support Document for the Cyanobacterial Toxin Cylindrospermopsin
US EPA Drinking Water Health Advisory for the Cyanobacterial Toxin Cylindrospermopsin
US EPA Drinking Water Health Advisory for the Cyanobacterial Microcystins Toxins
US EPA Harmful Algal Blooms and Seafood Safety
US EPA Presentations EPA Webinar Human Health Risks Associated with Cyanobacteria and Cyanotoxins Exposure, May 22, 2013
US EPA Presentations EPA Webinar Human Health Risks Associated with Cyanobacteria and Cyanotoxins Exposure, May 23, 2013
US EPA’s Fish and Shellfish Newsletter
SWAMP's California Freshwater Harmful Algal Bloom Field Guide - Health and Safety Guide Exit

For comments, feedback or additional information, please contact Lesley D'Anglada (, Project Manager, at 202-566-1125.

Top of Page